In order to increase the efficiency of solar absorbers, solar selective coatings are applied to the solar absorbers. Such coatings increase absorbance of solar radiation in a spectral range in which the solar radiation has high intensity and reduce loss of energy in the infrared spectral range.
Solar selective coatings for solar absorbers typically comprise a metallic layer on an absorber body, a solar absorptive coating and a top layer. Alternatively, the solar absorptive coating may be positioned directly on a metallic absorber body. The absorptive layer typically comprises a metallic component and a non-metallic component, such as a dielectric component. Typically the metallic material forms islands in the dielectric material so that a Cermet material is formed. The top layer has a metal concentration that is lower than that of the solar absorptive layer or is free of metal. Such a solar selective coating absorbs solar radiation while the emission of infrared radiation is reduced compared with, for example, a uniform metallic coating.
Solar absorptive coatings typically have a non-uniform metal volume fraction. FIG. 1 show plots of metal volume fraction versus depth from an outer boundary for an exemplary selection of such solar absorptive coatings. The figure shows a plot 10 for a coating having a graded metal volume fraction, a multilayer structure 12, and a combination of graded and multilayer profiles 14.
For example, a solar energy reflector array may be used to collect sunlight which is then focused onto the absorber coated with the solar selective surface coating. The collected sunlight heats the absorber and the solar selective coating locally to relative high temperatures such as 350° C. In order to increase the lifetime of the solar selective surface coating at such high temperatures, and to reduce thermal losses of the absorber, the absorber may be positioned in an evacuated housing.
It is known that the conversion efficiency of the energy from the collected sunlight is better at even higher temperatures, but further increase of the temperature may have a substantial negative impact on the lifetime of the solar selective coatings.
The positioning of the absorber with solar selective surface coating in air, which would be advantageous for some applications, can cause even more problems.
There is a need for technological advancement.